A 32-year-old man is being questioned following the seizure of 6kg of methamphetamine in Birr, Co Offaly.
It is the largest ever seizure of the drug in Ireland.
Officers say the synthetic drug is extremely dangerous and has already had a detrimental impact on the criminal justice and health services in the US.
The methamphetamine and a quantity of herbal cannabis were found concealed in the body work of two cars.
The drugs are estimated to have a street value of €500,000.
There have also been follow up searches in Cork and Kerry.
Gardaí say that due to the size of the haul, it is likely that an organised criminal gang is trying to establish a market for methamphetamine in Ireland.
Cannabis seizure
Elsewhere, five men were arrested in connection with the seizure of a number of cannabis plants and cultivation equipment in Sligo.
A number of searches were carried out in the Sligo/Leitrim area beginning on Sunday as part of Operation Clean Street.
Cannabis resin and cocaine were found at a number of locations.
Since November last year gardaí from the Sligo/Leitrim Divisional Drugs Unit, assisted by members from the Garda National Drugs Unit have been involved in the operation in the Sligo District.
The five men are due to appear before Sligo District Court on 31 July.
